The exocasting process can be used to form, for example, silicon photovoltaic substrates using a molten semiconducting material, such as molten silicon. A mold, such as one comprising refractory materials, may be dipped into and removed from the molten silicon to form a solidified layer of silicon on an exterior surface of the mold. The molten silicon is typically disposed within a vessel.
In various applications, it is desirable that the vessel used to hold the molten silicon be made from materials meeting particular exocasting requirements, such as high temperature stability and purity. By way of example, in at least some processes, the molten silicon is heated to temperatures of at least about 1412° C., and thus the vessel that contains the molten silicon is preferably made from a material that is able to withstand the high molten silicon temperature. In addition, in various embodiments, in order to form consistent silicon wafers without uneven surfaces, the crucibles can be made of a material of a sufficiently high purity so that impurities from the crucible do not contaminate the molten silicon. The vessel may also provide thermal insulation properties, for example, to maintain the silicon in a molten state.
The inventors have now discovered a process of making a unitary structure, such as a vessel for holding molten semiconducting material that is of sufficient temperature stability and purity, while also providing sufficient thermal insulating properties, for use in certain applications, such as various methods of making articles of semiconducting materials. It should be noted, however, that while the structures disclosed herein may have certain temperature stability, purity, and/or thermal insulating properties, as described, the temperature stability, purity, and/or thermal insulating properties of at least some embodiments may be present in a greater or lesser degree, or may not be present at all. However, such embodiments are intended to be within the scope of the disclosure.
The inventors have also discovered a process of making a vessel in the form of a unitary structure that may be formed from multiple structural layers, while minimizing the number of steps to form the layers, where the layers have sufficiently high purity and are sufficiently thermally insulating.